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@peter_mccc, Not sure if you saw a previous post of mine, but after posting screenprints of multiple license verification app failures, I did contact the developer using the suggested email and received replies that were cut and paste from their FAQ.
As I can't defend the app from deserving a one-star rating with this yet-to-be-addressed catastrophic failure mode, you will likely see one of the above posted replies from the developer to similar ratings and ratings comments on the app failure.

As @Growler mentioned doing, I too have looked at the reviews and only until recently, below was the cut and paste response for those reporting the app failure due to license check.
Max MP October 2, 2017
Google Play verifies license for Poweramp 2 times in first 24 hours after purchase/install. If it happens for you all the time, sorry but you have NO valid Poweramp purchase or your google account or Play setup is wrong. Please write to Poweramp.maxmpz@gmail.com or check Poweramp in-app support. Thanks!
The replies now are a more innocuous cut and paste requesting to report the issue. But can anybody report that they have had a reply from Max after writing with questions or concerns? I have received a reply each of the 3 times I've written from someone named Alex, but they are cut and paste from the help section and/or FAQ and did not address the issue.
Max MP
Can you please write your issue details to Poweramp.maxmpz@gmail.com?

@andrewilley,
At next opportunity, after using the App for approx. 30 to 45 minutes in airplane mode, try moving to the next track though about 15-20 songs without them really even starting. That seems to provoke the failure for me.

@andrewilley
In your attempts to duplicate the issue, have you disabled Google Play's access to cell/mobile data? I'm quite sure that is why the failures are occurring. Granted, it doesn't happen right away, but in my case, it has yet not to fail.
I also removed cell/mobile data access from Poweramp as well, but don't believe that plays a role based on the error generated being a Google Play "ERROR_CONTACTING_SERVER".

@andrewilley
I understand the FAQ* states the license isn't checked again, but the data doesn't seem to support that as I have shown through multiple uninstalls/reinstalls that after installing the Unlocker, Poweramp shows verified and if the FAQ statement is correct, then it shouldn't check again (except under 3 very specific instance, each being rare occurrences). But many users report that it does indeed fail at a later date and well after the initial verification has occurred.
It is much more plausible that whether by intent or not, the licensing check does occur after the initial verification and if there is not an internet connection for the license check, (which happens to occur through Google Play which is unlikely to be the cause of the issue) then the [re]verification fails and Poweramp fails. This is a much more plausible issue which has yet to be addressed. *Poweramp FAQ:
Is Internet connection required?
← Poweramp F.A.Q.
Internet connection is not required for normal operation.
Internet connection is required for a short time after Poweramp purchase.
Poweramp (main app):
- uses optional Internet connection for Album art downloading, and Support.
Poweramp Unlocker:
- for Play purchase license is checked via Internet in first 24 hours after Unlocker installation and Poweramp next start, and if the Google Play Store check is successful, then it's never checked again, unless Poweramp is reinstalled, system is reset, or ROM is updated
- for Website purchase, license is checked via Internet once after Unlocker installation and Poweramp next start, and if the check is successful, then it's never checked again, unless Poweramp is reinstalled, system is reset, or ROM is updated
If license check is failed, Poweramp will try to check it until it fails completely in few attempts.

@andrewilley,
Thank you for checking into this and for moderating the forum.
Is it your understanding that Poweramp does not continue to check the licence status after the initial unlocker install/licence check? Data from my experiences shows that it does continue to check it. And if there is no internet connection at the time, the licence verification will fail and the App will shut down/fail. If the App does not continue to validate the licence, then it would be difficult to assign cause for the continued Poweramp failures on which rev of Android and/or Google Play is/was running.
Also found this interesting. Just what version of Android is Poweramp written/optimized for? I am running 7.0 (Nougat)
I received a reply from Alex from Poweramp Tech Support as mentioned in my post yesterday and was disappointed, but I guess not surprised, to find out it's an exact cut and paste from the previous reply: On Tuesday, July 3, 2018, 2:19:21 AM GMT, Alex Platonov <Poweramp.maxmpz@gmail.com> wrote:
Hello from Poweramp support team!
Please uninstall Poweramp Trial and Unlocker, restart your device and update Poweramp (trial - which is the main app) from Poweramp site -http://powerampapp.com/download-Poweramp to latest build.
Thanks!
***************************************************************************************************
On Friday, June 29, 2018, 3:30:07 AM GMT, Alex Platonov <Poweramp.maxmpz@gmail.com> wrote:
Hello from Poweramp support team!
Please uninstall Poweramp Trial and Unlocker, restart your device and update Poweramp (trial - which is the main app) from Poweramp site -http://powerampapp.com/download-Poweramp to latest build.
Thanks!

My latest failure iteration after following advice from Poweramp Tech Support to delete and reinstall the App:
Hello Alex,
I followed your advice and removed both Poweramp and the Unlocker from the device. Then downloaded and installed Poweramp trial from your website followed by installing the Unlocker from Play. As seen below, the App reports that the full version was Purchased and Verified.
After using the App for just a couple of days, it reported the error "Too many failed files. Playing will now stop." as can be seen in the screenprint. This was followed by the well reported Google Play error "ERROR_CONTACTING_SERVER" followed by the Failure to verify the Licence.
I can confirm there was no internet service available during the time of the Poweramp App use, approximately 30 minutes, which at that time, the App failed. The understanding is that once the purchase and licence is verified, that the App does not check again. Is that correct, as it does not appear to be the case and does indeed attempt to verify the licence again and again during App usage.
Pls offer a resolve to this via patch or the likes.

Update: I did contract the developers @ Poweramp.maxmpz@gmail.com and received a prompt reply from Alex. The first suggestion was to delete and reinstall the unlocker. Of course this is what I have done in the past each time the app fails with the licence check and so made this known. The follow up was to uninstall the unlocker and the trial version and reinstall from their website as opposed to google play and then follow up with the unlocker. It was the same rev of app, so not sure what if anything will be different, but shall see.
Will report back if issue reoccurs and admittedly, I have little faith that the suggestions will have resolved the issue. Even if so, it is a considerable inconvenience and the app bug should be correctly resolved with an update.

Have experienced this app failure multiple times and all failures had this is common: Default for Google Play is to access via Wifi (internet) only (i.e. mobile data restricted) / Wifi not enabled (at the time of occurrence) / After app has been running for some time, rapidly advance to next song ~15 times w/o any song actually starting to play / get some 'file' related error / app fails with license validation.
Occurs while out on long bike rides and results in resorting to default music player. This bug should garmer much more attention than it does given the age of this particular thread.
Very good app but would not recommend, and shouldn't be marketed without at least a disclaimer, until this app-disabling bug is resolved. @maxmp